FRICTION

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FRICTION

• Chapter 3
• Section 2

2
Friction

• Suppose you decide to ride a skateboard.
• You push off the ground and start moving.
• According to Newtons First Law of Motion, if no
  other forces are acting on your skateboard, you
  will continue to move.BUT
• What happens?
• You slow down

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Friction

• Your skateboard slows down because of friction.
• Friction is the force that opposes motion between
  two surfaces that are touching each other.
• The amount of two surfaces depends on two things
• Kind of surface
• Forces pressing the surfaces together

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What Causes Friction?

• Even though a surface may look smooth, if you
  magnify the surface it isnt smooth.
• Everything is made of molecules. The molecules
  come together to make bumps. Some surfaces have
  molecules that leave large bumps and some leave
  smaller bumps, but all surfaces have bumps.
• Microwelds occur when two bumpy surfaces are
  rubbed up against each other they stick together.

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Sticking Together

• The greater the force on the object, the greater
  the force of the microwelds and the greater force
  is needed to overcome the microwelds to move the
  object.

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Static Friction

• Suppose your kid brother gets into a box and
  wants you to move him. He is too heavy to lift.
• You try to push the box and it doesnt move.
• What type of acceleration does it have?
• Zero Acceleration

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Static Friction

• Remember Newtons Second Law of Motion
• If the acceleration is zero, the net force is
  zero.
• Therefore, because you cant move it, another
  force is being applied to cancel out your force.
• That opposing force is friction caused by the
  microwelds between the bottom of the box and the
  floor.
• This type of friction is called static friction.

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Static Friction

• Static Friction is the friction between two
  surfaces that are not moving past each other.
• In this case, your push is not large enough to
  break the microwelds and the box remains stuck to
  the floor.

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Sliding Friction

• To help you move your kid brother, you decide to
  ask a friend.
• You both push and slowly, but not easily, the box
  starts to slide across the floor.
• When you stop pushing, it quickly stops moving.
• Sliding the box is difficult and there is still
  resistance.
• Although you have overcome the microwelds, there
  is still a sliding friction occurring.

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Sliding Friction

• Sliding Friction is the force that opposes the
  motion of two surfaces sliding past each other.
• Sliding friction is caused by microwelds
  constantly breaking and then forming again as the
  box slides along the floor.
• To keep the box moving, you must continually
  apply a force to overcome sliding friction.

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Rolling Friction

• Have you ever seen a car stuck in the snow. Every
  time the driver steps on the gas, the tires just
  spin and the car doesnt go anywhere?
• This is because there isnt enough friction
  between the slippery ground and the tires.
• How could you create more friction to get the
  tires to move the car?
• Drop sand, gravel, put a board under each of the
  front tires.

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Rolling Friction

• The friction between the rolling tires and the
  ground is called rolling friction.
• Rolling friction works partly because of
  microwelds.
• Rolling friction is much less friction than
  static or sliding friction.
• That is why it is easier to use a dolly than
  pushing your kid brother in the box.

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Friction

• Why do some people add sand to their trunks
  during the winter months?
• How does this apply to any of Newtons Laws?